The present invention concerns an apparatus for monitoring the pressure obtaining in a compressed air line of an installation comprising in particular components which are actuated pneumatically and possibly components which are fed by other forms of energy, for triggering off a safety operation (for example cutting off a feed) in respect of that installation if the pressure being monitored falls below a certain threshold which is defined by the user of the line in dependence on the safety factor that he intends to provide.
Safety regulations and practice in regard to construction and operation of automatic machines require that, after an interruption in the supply of energy, whether it is electrical or pneumatic, starting a machine again is the subject of a voluntary and deliberate action. Moreover a drop in pressure in a feed line to an installation may result in defective operation of some machines (poor feed in respect of the tools, carrier members, transfer members and the like), which can even be dangerous from the point of view of the operating personnel.
It is therefore important to be able to cut off the feed to an installation in the event of a drop in pressure below a given threshold, and it is also important for that interruption in the feed to be maintained, in spite of the pressure rising again, until an operator performs a deliberate action.
There are pneumatic threshold valves which cut off the intake of fluid under pressure when the pressure falls for example below two bars and which have to be brought into operation again by a manual re-arming step. The pressure threshold in those apparatuses is generally not adjustable and structurally the exhaust is limited in respect of section so that the operation of purging the installation, after an interruption has occurred, is a long one. In addition, their present design is poorly suited to the variation in needs and to the requirements in respect of reliable operation. Also known are manostats which, at a given adjustable pressure threshold, supply an electrical signal which on the one hand may be used on some component of the machine and which on the other hand can feed an electrically operated valve which provides for pilot control of a valve for controlling all or part of the compressed air feed circuit. That assembly involves a high level of cost and is laborious to install. In addition its level of reliability is open to criticism because of the different conversions of pneumatic parameters into electrical parameters and vice-versa.